1. Field Of The Invention
The present invention is directed to drilling fluid processing systems; shale shakers; to methods for using these things; and, in certain particular aspects, to the separation of lost circulation material from used drilling fluid.
2. Description of Related Art
In the oil and gas industries, shale shakers use screens to treat drilling fluid contaminated with undesirable solids. Typically such apparatuses have a basket, deck, or other screen holding or mounting structure mounted in or over a receiving receptacle or tank and vibrating apparatus for vibrating one or more screens. Material to be treated is introduced to the screen(s) either by flowing it directly onto the screen(s) or by flowing it into a container, tank, or “possum belly” from which it then flows to the screen(s). Often, the screen or screens used to treat material is sealed in place on a screen deck, in a screen basket, or on screen mounting structure.
In the past it has been common to use multiple screens at multiple levels in a shale shaker to process drilling fluid, e.g., screens at one, two or three levels.
“Lost circulation” of drilling fluid occurs when, in drilling a wellbore, the circulation of drilling fluid to and then away from the drill bit ceases due to the porosity of the formation and/or due to fracturing of the formation through which the wellbore is being drilled. When lost circulation occurs, drilling fluid is pumped into the fractured formation rather than being returned to the surface. Often circulation is lost at some specific depth where the formation is “weak”, and that the fracture extends horizontally away from the borehole. Expressions used to describe rocks that are susceptible to lost returns include terms like vugular limestone, unconsolidated sand, “rotten” shale, and the like.
A wide variety of “lost circulation materials” (“LCM”) have been pumped into wellbores to fill or seal off a porous formation or to fill or seal off a wellbore fracture so that a proper route for drilling fluid circulation is re-established. Often lost circulation materials are generally be divided into fibers, flakes, granules, and mixtures.
Often it is also desirable to recover and retain the lost circulation material in the drilling mud system during continuous circulation. Screening the drilling mud for removal of undesired particulate-matter can also result in removal of the lost circulation material and, therefore, require continuous introduction of new lost circulation material to the drilling mud downstream of the mud screening operation.
The addition of lost circulation material compounds the separating problems because it, like the drilling fluid, is preferably cleaned and recirculated. Exiting the well is the drilling fluid of small size, the lost circulation material of a large size, and the undesirable material of a size therebetween, with the largest and smallest of the materials to be recirculated. One proposed solution to this separation problem is a conventional two step screening process as shown in U.S. Pat. No. 4,116,288. There the exiting mixture of drilling fluid, lost circulation material and undesirable material is first subjected to a coarse screening to separate the lost circulation material from the drilling fluid and undesirable material which drops to a second finer screen therebelow to separate the drilling fluid from the undesirable material. The drilling fluid and lost circulation material are then reunited for recirculation into the well. This system is susceptible to height restrictions and fine screen problems. The lost circulation material can be coated with undesirable material which will not go through a first screen, moves over and exits off of the top side of the first screen, and is circulated back into a well.
There are a variety of known drilling fluid processing systems, shale shakers, and methods for recovery of lost circulation material; including, for example, but not limited to, those in U.S. Pat. Nos. 6,868,972; 6,669,027; 6,662,952; 6,352,159; 6,510,947; 5,861,362; 5,392,925; 5,229,018; 4,696,353; 4,459,207; 4,495,065; 4,446,022; 4,306,974; 4,319,991; and 4,116,288 (all said patents incorporated fully herein for all purposes).
In certain prior systems, problems have been encountered with systems for screening out lost circulation material when undesirable material of the same size is also screened out.